The present invention is directed to a method for the electrolytic etching of a recrystallized aluminum foil and the application of the foil.
In electrolytically etching aluminum foils, an enlargement of a surface area will occur. As a result of this enlargement of the surface area, electrolytic capacitors, which are manufactured with the etched foils, can obtain a given capacity with a smaller volume than an electrolytic capacitor, which utilizes a foil with the non-roughened surface or unetched surface.
Aluminum foils, which have been roughened according to previously known etching methods or techniques, exhibit in addition to the regular tunnel-like etching structure, undesirable overlays of irregular pitting which pitting is structurally too coarsely porous. As a result, only about 10-60% of the mathematically possible capacitance multiplication factors, which are obtained with an idealized etching structure, can be obtained in relation to the aluminum electrolytically stripped from the etched foil. The ideal etching structure is a pure tunnel etching with defined and uniform tunnel diameters and without any undesirable pitting of the foil.
It is known to achieve pure tunnel etching structures without coarse pitting in recrystallized aluminum by utilizing a very low etching current density in the magnitude of 10 mA/cm.sup.2. However, the tunnel densities, which are achieved with the low current densities that are utilized with known etching techniques, fall extremely short of a required value of &gt; 10.sup.7 tunnels/cm.sup.2 of foil surface, which value is required to achieve a high surface capacitance multiplication factor.
It is indeed further known that tunnel density can be increased with an increased etching current density. However, this is only possible when one will accept an increasing coarser and more irregular pitting of the aluminum foil. The known etching methods are therefore a compromise between the highest possible tunnel density and a relatively tolerable portion or amount of irregular pitting that is structurally too large. Therefore, the limits of the possible roughening factor with given foil thicknesses and aluminum erosion rates are far from being obtained particularly for aluminum foils which are employed in low voltage electrolytic capacitors.